Olefins, particularly light olefins, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming an alternative feedstock for making light olefins, particularly ethylene and propylene. Promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including synthesis gas derived from natural gas; petroleum liquids; and carbonaceous materials, including coal. Because of the relatively low-cost of these sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
One way of producing olefins is by the catalytic conversion of methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 to Kaiser, discloses making olefins from methanol using a variety of SAPO molecular sieve catalysts. The process can be carried out at a temperature between 300° C. and 500° C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hr−1.
Inui (J. Chemical Society Chem. Commun. p. 205, 1990) has shown that the selectivity to ethylene can be increased when methanol is contacted with a nickel-substituted SAPO-34 rather than an unsubstituted SAPO-34. In this case, nickel substitution occurred into the SAPO-34 framework.
In contrast to the work of Kaiser and Inui, metal incorporation may also take place post-synthesis, that is, following the synthesis of the molecular sieve framework. For example, U.S. Pat. No. 5,962,762 to Sun et al. teaches a process for converting methanol to light olefins using a metal-incorporated SAPO catalyst. The catalyst is produced by allowing a SAPO molecular sieve to remain in contact at ambient conditions with an aqueous metal solution, preferably a nickel or cobalt containing solution, whereby the metal is adsorbed onto the sieve. The treated molecular sieve is then separated from the solution and dried. U.S. Pat. Nos. 5,625,104 and 5,849,968 to Beck at al. teach a process of incorporating alkali and alkaline earth metals into a zeolitic catalyst by pretreating the zeolite with an organosilicon or poly-oxo silicon compound followed by the treatment with a metal solution. U.S. Pat. No. 4,692,424 to Le Van Mao teaches a process for the dry incorporation of manganese ions on the external reactive sites of zinc-containing ZSM-5 and ZSM-11 by adding a minimum amount of an aqueous manganese solution to form a malleable paste and extruding the paste under pressure.
Post-synthesis metal incorporation of zeolite catalysts is also used for other processes. For example, U.S. Pat. No. 6,084,142 to Yao et al. teaches treating a ZSM-5 catalyst with a solution of a zinc component, such as dimethylzinc, followed by steam treatment for the conversion of hydrocarbons to aromatics and lower olefins. There is no teaching of conversion of methanol to olefins.
Yamamoto et al. (Microporous and Mesoporous Materials 44–45, Organic Functionalization of Mesoporous Molecular Sieves with Grignard Reagents, p. 459–464, 2001) teach post-synthesis organic functionalization of MCM-41 in a two step procedure. MCM-41 is first modified by alcohols, which leads to the esterification of surface silanol groups (converting Si—OH to Si—OR) and then allowed to react with a Grignard reagent R′MgX which converts Si—OR to Si—R′. Again, there is no teaching of conversion of methanol to olefins.
PCT Application WO 97/26989 teaches a process for producing a catalyst by combining a medium pore, non-zeolitic molecular sieve, such as SAPO-11, SAPO-31 and SAPO-41, with an active source of a hydrogenation component in a non-aqueous solvent. The resultant catalyst is disclosed as being useful for hydrocracking and catalytic dewaxing. There is no teaching of conversion of methanol to olefins.
Although much research has already been undertaken to optimize aluminophosphate and silicoaluminophosphate molecular sieve catalysts for use in the conversion of methanol to light olefins, there remains a need to develop catalysts which show improved selectivity to the desired olefins, ethylene and propylene, and in some cases improved selectivity to ethylene alone. Moreover, since methanol conversion catalysts tend to undergo rapid catalyst deactivation due to the formation of coke, thereby requiring frequent regeneration, there is also a need for catalysts which deactivate more slowly, that is have a longer effective lifetime.